Pulsed doppler detection system

ABSTRACT

Apparatus is herein disclosed for a pulsed doppler detection system comprising apparatus for generating pulsed pairs including a coherent pulsed oscillator wherein the return from target first pulse of a pair is mixed with the then currently generated second pulse of the same pair within the oscillator when the range delay is correct, and further including means for jittering the pulse pair to decorrelate out of range signals.

United States ate i Inventor Benjamin Palleiko Norwood, Mass. Appl. No.809,263 Filed Mar. 21, 1969 Patented Sept. 7, 1971 Assignee SandersAssociates, Inc.

Nashria, NH.

PULSE!) DOPPLER DETECTION SYSTEM [56] References Cited UNITED STATESPATENTS 3,320,612 5/1967 Crow et al. 343/17.1 UX 3,329,952 7/1967 Bogle343/7 PF 3,353,179 11/1967 Cartwright 343/12 MDI 3,491,360 1 1970Stoorvogel "I. 343/17.1PRF

Primary Examiner-T. H. Tubbesing Attorney-Louis Etlinger ABSTRACT:Apparatus is herein disclosed for a pulsed doppler detection systemcomprising apparatus for generating 9 Claims 3 Drawing Figs pulsed pairsincluding a coherent pulsed oscillator wherein the US. Cl 343/17.l,return from target first pulse of a pair is mixed with the'then 343/5PD, 343/7 PF, 343/12 MD currently generated second pulse of the samepair within the Int. Cl GOls 7/28 oscillator when the range delay iscorrect, and further includ- Field of Search 343/7 PF, ing means forjitter-ing the pulse pair to decorrelate out of 12 MD, 17.1 rangesignals.

J 'L P DOUBLE COHE RENT SIGNAL GES ESETOR fifii R K C GENERATOR O 0 22l2 l4 l8 24 DOPPLER a DOPPLER AMPLIFIER FILTE PULSED DOPILER DETECTIONSYSTEM BACKGROUND OF THE INVENTION There are many applications whichrequire detection of a target at a predetermined distance (range), forexample, missile or bomb fuzing, personnel detectors or near missindicators. In the past these type applications employed CW radarsystems or conventional pulse systems. The CW systems are relativelysimple but are sensitive to interfering signals and lack good rangecutoff characteristics. The conventional pulse systems, which requiredual antennas or RF switching and separate mixers, provide good rangecutoff characteristics, however are relatively complex.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a simplified pulsed doppler detection system.

It is another object of this invention to provide a pulsed dopplerdetection system having a combined oscillator, local oscillator andmixer.

It is a further object of this invention to provide a pulsed dopplerdetection system employing double pulsing wherein the second pulse ofthe pulse pair becomes the local oscillator pulse for the return fromthe first pulse of the pulse pair.

Briefly, a pulsed oscillator detection system is provided comprisingapparatus for generating pulse pairs at a predetermined FRF, including acoherent pulsed oscillator wherein the return first pulse of a pulsepair is mixed with the second pulse being generated therein when desiredtarget range occurs. The system further includes means for jitteringpulse pairs to eliminate range ambiguities.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other featuresand objects of this invention will become more apparent by reference tothe following description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a pulsed doppler detection system;

FIG. 2 are sketches of simplified waveforms as generated by the systemof FIG. ll; and

FIG. 3 is a schematic illustrating detailed circuitry for one embodimentof the block diagram of FIG. ll.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIGS. I and 2,there is illustrated thereby an embodiment of a pulsed doppler detectionsystem. A pulse train, such as signal 110 of FIG. 2, is generated by aPRF generator 12. Signal has a predetermined PRF (pulse repetitionfrequency). The signal It) is preferably jittered in a random orpseudorandom manner to eliminate range ambiguity (not illustrated forclarity in FIG. 2). This jittered signal is applied as the input signalto a double pulse generator 14 which generates a pulse pair for eachinput pulse. The pulse pair separation range gates the system. Sinceeach input signal to the double pulse generator is jittered, thecorresponding pulse pair will be jittered in like fashion. The outputsignal from pulse generator M is illustrated by signal 16 of FIG. 2 withjitter being omitted.

The output from double pulse generator 14 is applied to a coherentpulsed oscillator 18 for generating pairs of coherent RF pulses such assignal 20 of FIG. 2.

Pulsed oscillator 18 is coherent in that it has a predetermined startingphase.

The output of coherent pulsed oscillator 18 is transmitted by an antenna22 which also acts as the receiving antenna.

The returned echo signal from a target is applied to coherent pulsedoscillator 18. When the range delay is correct, the target return fromthe first pulse of each pair mixes with the second pulse being currentlygenerated in the oscillator, thus, providing a beat frequency dopplersignal at an output 24. The second pulse of a pulse pair acts as thelocal oscillator for the return from the first pulse of the pair. Thepulse pair is preferably jittered relative to preceding pulse pairs toreduce second time around signals and interfering signals. Note that thedoppler signal may be extracted directly.

The return from previous pulses may, for certain ranges, fall directlyon pulses of subsequent pairs being generated. However, the jitter onthe PRF decorrelates these pulses to prevent the generation of dopplersignals. Interfering signals are likewise decorrelated by the PRFjitter.

The doppler signal produced in oscillator 18 is filtered by a dopplerfilter 26 and amplified by an amplifier 28 with the resultant signal 30being applied to a utilization means, for example, trigger circuits.

The doppler filter eliminates out'of-band signals and PRF components.

Referring now to FIG. 3, there is thereby illustrated one embodiment ofthe detailed circuitry for the elements illustrated in block form inFIG. 1.

The basic PRF of the system is generated by a PRF generator 32 which inthis embodiment comprises a pair of transistors 34, 36 which form afree-running (astable) multivibrator. The PRF is determined by resistors38, 40, 42 and capacitors 44, 46.

A Colpitts oscillator is employed as the jitter source for PRF generator32. A transistor 50 forms the oscillator with the frequency thereofdetermined by a pair of capacitors 52, 54 and an inductor 56. The outputfrom the jitter source 48 is applied via a capacitor 58 to frequencymodulate the PRF on multivibrator 32. If desirable, noise jitter from anoise source (not shown) can be applied at point 60.

The output signal from PRF generator 32 is differentiated by a capacitor62 and a resistor 64 and applied to a doublepulse generator 66.

Double-pulse generator 66 includes a monostable multivibrator 68comprising transistors 70, 72 which receives the differentiated signalas a trigger thereto. When triggered, multivibrator 68 generates asingle-pulse of length determined by a resistor 74 and a capacitor 76.The duration of this pulse determines the length of the RF pulse to betransmitted.

A transistor 78 provides power gain for the output signal frommonostable multivibrator 68. This signal drives a delay line 80 and atransistor 82, simultaneously. When the pulse reaches the end of delayline 80 it is reflected and returns to the input of delay line 80; thereit drives transistor 82 a second time, thus producing a double-pulseinto transistor 82.

When transistor 82 is driven on, it connects a transistor 84 of coherentoscillator 86 to the supply voltage-V.

If the pulse rise time is sufficiently fast, it produces a coherent RFpulse. Inductor 88 and capacitor 90 determine the oscillator frequency.As previously mentioned, the RF pulse duration is determined by themultivibrator 68. This RF pulse output taken at point 92 is transmittedfrom the antenna (not shown).

Target returns from the correct range arrive at the oscillator when thesecond pulse of the pair is being generated. This return pulse beatswith the second pulse, producing a modulated video signal at the emitterof transistor 84. An RF choke 94 couples out the doppler components fromthe oscillator.

In an alternate embodiment, only a signal pulse is employed. In whichcase, the target return beats with the trailing end of the pulse stillbeing generated in the oscillator.

While I have described above the principles of my invention inaccordance with specific apparatus, it is to be clearly understood thatthe description is made only by way of example and not as a limitationof the scope of my invention as set forth in the accompanying claims.

Iclaim:

1. A pulsed doppler detection system, comprising:

means for generating a first signal comprising pulse pairs of apredetermined pulse repetition frequency;

means for transmitting said signal;

means for receiving a reflected signal from said transmitted signal; and

means for generating a (second) doppler signal when the reflectedpulsefrom a first pulse of a pulse pair is received coincident with thegeneration for the second pulse of said pulse pair, said reflected firstpulse of a pair being mixed with said second pulse of said pair.

2. Apparatus as defined in claim 1, in which said means for generating afirst signal includes:

a signal generator for generating a pulse train at said predeterminedpulse repetition frequency;

a double-pulse generator coupled to said signal generator for generatinga pulse pair for each pulse received from said signal generator, saidpulse pair havinga predetermined spacing between pulses; and

a pulsed oscillator coupled to said double-pulse generator forgenerating an RF pulse for each pulse received from said double-pulsegenerator.

3. Apparatus as claimed in claim 2, in which said transmitting meansincludes an antenna coupled to said pulsed oscillator.

4. Apparatus as claimed in claim 3, in which said pulsed oscillator iscoherent, each RF pulse generated thereby having the same startingphase.

5. Apparatus as defined in claim A, in which said means for generating adoppler signal includes means for coupling a second output from saidpulsed oscillator.

6. Apparatus as defined in claim 5, in which said means for generating adoppler signal further includes a filter coupled to said second outputof said pulsed oscillator.

7. Apparatus as defined in claim 6, in which said means for generating adoppler signal further includes an amplifier coupled to said filter.

8. Apparatus as defined in claim 5, further including means forjittering each of said pulse pairs.

9. A pulsed doppler detection system, comprising:

means for generating an RF pulse including an oscillator;

means for transmitting said pulse;

means for receiving a reflected signal from said transmitted signal; and

means for generating a doppler signal in said oscillator when thereflected pulse is received coincident with the generation of the latterportion of said generated pulse.

1. A pulsed doppler detection system, comprising: means for generating afirst signal comprising pulse pairs of a predetermined pulse repetitionfrequency; means for transmitting said signal; means for receiving areflected signal from said transmitted signal; and means for generatinga (second) doppler signal when the reflected pulse from a first pulse ofa pulse pair is received coincident with the generation for the secondpulse of said pulse pair, said reflected first pulse of a pair beingmixed with said second pulse of said pair.
 2. Apparatus as defined inclaim 1, in which said means for generating a first signal includes: asignal generator for generating a pulse train at said predeterminedpulse repetition frequency; a double-pulse generator coupled to saidsignal generator for generating a pulse pair for each pulse receivedfrom said signal generator, said pulse pair having a predeterminedspacing between pulses; and a pulsed oscillator coupled to saiddouble-pulse generator for generating an RF pulse for each pulsereceived from said double-pulse generator.
 3. Apparatus as claimed inclaim 2, in which said transmitting means includes an antenna coupled tosaid pulsed oscillator.
 4. Apparatus as claimed in claim 3, in whichsaid pulsed oscillator is coherent, each RF pulse generated therebyhaving the same starting phase.
 5. Apparatus as defined in claim 4, inwhich said means for generating a doppler signal includes means forcoupling a second output from said pulsed oscillator.
 6. Apparatus asdefined in claim 5, in which said means for generating a doppler signalfurther includes a filter coupled to said second output of said pulsedoscillator.
 7. Apparatus as defined in claim 6, in which said means forgenerating a doppler signal further includes an amplifier coupled tosaid filter.
 8. Apparatus as defined in claim 5, further including meansfor jittering each of said pulse pairs.
 9. A pulsed doppler detectionsystem, comprising: means for generating an RF pulse including anoscillator; means for transmitting said pulse; means for receiving areflected signal from said transmitted signal; and means for generatinga doppler signal in said oscillator when the reflected pulse is receivedcoincident with the generation of the latter portion of said generatedpulse.